This invention relates to an optical instrument and more particularly to an optical instrument employing tilting filters to scan the wavelength of light transmitted to a measurement subject through a spectral range.
In U.S. Pat. Nos. 3,861,788 to Donald A Webster and 4,082,464 to Robert A. Johnson, there are disclosed optical instruments designed to measure reflectivity or transmissivity of a measurement subject by directing light with a very narrow wavelength band onto the measurement subject and measuring the resulting light emanating from the measurement subject such as by reflection by or transmission through the measurement subject. In the instruments, a plurality of interference filters are mounted on a filter wheel and each filter is rotated by the filter wheel sequentially into the path of collimated light in the near infrared range. As the filter moves through the beam of collimated light, the angle of the filter to the instrument light varies and the narrow bandwidth of light transmitted by the filter is changed. In this manner the center wavelength of th narrow wavelength band transmitted to subject is swept through a selected spectrum, specifically, the near infrared spectrum.